1. Field of the Invention
The present invention relates to a projection exposure apparatus and a method for correcting a positional discrepancy of a projected image, used to expose a liquid crystal display substrate with a pattern by using the projection exposure apparatus. In particular, the present invention relates to a scanning type projection exposure apparatus for illuminating different segmental areas of a pattern area on a mask respectively, and projecting images of the respective segmental areas onto exposure areas (areas to be exposed) on a substratethrough a plurality of projection optical systems respectively. Further, the present invention relates to a method for correcting positional discrepancies of images of respective segmental areas, and a method for determining image formation characteristics of projection optical systems.
2. Description of the Related Art
A projection exposure apparatus, in which a photosensitive substrate is exposed with a pattern formed on a mask, by means of transfer through a projection optical system, has been hitherto used in photolithography steps for producing semiconductor elements and liquid crystal display substrates. The projection exposure apparatus includes those of the so-called step-and-repeat system and the mirror projection system.
Recently, it is demanded to increase the areal size of the liquid crystal display substrate, in accordance with which it is desired to enlarge the exposure area of the projection exposure apparatus. A scanning type exposure apparatus provided with a plurality of projection optical systems has been developed as a measure for enlarging the exposure area. Namely, in such a scanning type exposure apparatus, a light beam radiated from a light source is allowed to pass through an optical system comprising, for example, a fly's eye lens to uniformalize its amount of light, and then the light beam is shaped into a desired shape by using a field diaphragm to illuminate a pattern plane on a mask therewith. A plurality of illuminating optical systems each having the arrangement as described above are disposed, and different segmental areas (illumination areas) on a mask are illuminated with light beams radiated from the plurality of the respective illuminating optical systems respectively. The light beams having passed through the mask form images of the pattern on the mask on different projection areas on a glass substrate through the different projection optical systems respectively. The entire plane of the pattern area on the mask is transferred onto the glass substrate by synchronously scanning the mask and the glass substrate with respect to the projection optical systems.
In general, the projection exposure apparatus is used such that a considerable number of layers are repeatedly exposed with original patterns while applying predetermined process treatments to one sheet of the glass substrate. The glass substrate is expanded or contracted (stretched) due to the process treatments (especially due to heating), and it is deformed from its initial state. The conventional exposure apparatus of the step-and-repeat system has only one projection optical system, in which the expansion or contraction (stretching) of the glass substrate may be corrected (corrected with magnification) by changing the projection magnification of the projection optical system, and changing the stop position of a stage during stepping so that a spacing distance between adjacent transferred images is changed. In the case of the exposure apparatus of the mirror projection system, the magnification in a scanning direction may be corrected by continuously changing relative positions of a master plate and a photosensitive substrate with respect to a projection optical system during scanning exposure, and the magnification in a direction perpendicular to the scanning direction may be corrected by changing the magnification of the projection optical system.
However, when the scanning type exposure apparatus provided with a plurality of projection optical systems as described above is used, a continuous pattern on a mask is divided and projected onto a glass substrate by using the plurality of projection optical systems so that divided images are formed without any gap, or they are overlapped with each other by predetermined amounts. Accordingly, the divided images are not continuously formed if there is a large difference between image formation characteristics of the respective projection optical systems as a matter of course, as well as if the mutual positional relationship between the projection optical systems is not in an expected relationship. Therefore, no technique equivalent to those hitherto used can counteract the stretching of the substrate.
In order to overcome such a problem, the present applicant has previously proposed, in Japanese Patent Laid-open No.7-183212, a scanning type exposure apparatus provided with an alignment means for a substrate and a mask, a means for adjusting image formation characteristics of respective projection optical systems, and a means for mutually calibrating the projection optical systems.
According to the scanning type exposure apparatus described above, the positional discrepancy in the X direction (scanning direction) between a substrate and a mask, the positional discrepancy in the Y direction (direction perpendicular to the scanning direction) therebetween, the rotation (rotation about the Z axis perpendicular to the XY plane), and the stretching of the substrate in the X direction can be accurately corrected by moving stages or changing the magnifications of the projection optical systems together with control of movement velocities of the stages while managing all of them by using position sensors such as laser interferometers for monitoring positional displacements of the mask stage and the substrate stage. Further, Japanese Patent Laid-open No.7-183212 discloses a method for counteracting the stretching of a substrate in the Y direction by providing a parallel plane glass piece between a projection optical system and a photosensitive substrate, and rotating the parallel plane glass piece so that the position of an optical axis of the projection optical system is deviated in the Y direction.
However, a first inconvenience arises. Namely, even when the method, which is based on the use of the technique disclosed in Japanese Patent Laid-open No.7-183212, is individually applied to counteract the stretching of the substrate in the X direction and the stretching of the substrate in the Y direction, it is difficult to adjust the image formation position of each of the projection optical systems with a high degree of accuracy in response to actual stretching of the substrate. Especially, it is impossible to correct the stretching of the substrate in the Y direction and the orthogonality error of the substrate caused thereby, while performing management by using position sensors such as laser interferometers.
When the image formation characteristic of each of the projection optical systems is adjusted according to the Japanese Patent Laid-open No.7-183212 described above, for example, it is assumed to use a mask stage comprising driving mechanisms and laser interferometers corresponding to one axis in the X direction and two axes in the Y direction, wherein the mask stage is driven, for example, in the Y direction. If driving amounts brought about by the two driving mechanisms are different from each other due to any cause, and hence different results of detection by the two laser interferometers are obtained, then a result of detection by the laser interferometer in the X direction necessarily changes. In such a situation, a second inconvenience arises. Namely, it is extremely complicated to calculate and determine, in real-time, the position of the mask stage on the basis of the results of detection obtained by the three laser interferometers.
Especially, a problem arises in the case of the use of a scanning type exposure apparatus comprising a mask stage including first reference marks used for positioning, the mask stage being driven by a plurality of driving mechanisms, a plurality of position detectors for detecting positional information on the mask stage, a ]-shaped (like a shape of staple) carriage for carrying the mask stage and a photosensitive substrate thereon, the carriage including second reference marks at positions corresponding to the first reference marks, and photoelectric detectors for detecting photoelectric signals corresponding to images obtained by projecting the first reference marks onto the second reference marks through projection optical systems, because of the following reason. Namely, it is necessary to determine image formation characteristics of the respective projection optical systems on the basis of a position of the mask stage and detected values obtained by the respective photoelectric detectors. Accordingly, it is indispensable to provide a plurality of central processing units (CPU's), and the detecting system depends almost only on the hardware. Consequently, the cost increases, and the system is complicated.